In the transportation industry, more emphasis has been put on recycling paved surfaces because of environmental concerns and the need to conserve natural resources. Perhaps because of that emphasis, asphalt has become the most recycled material in the United States. Typically, more than 70 million metric tons are recycled every year. Using recycled materials for roads is important to citizens, cities, and states throughout the country because it may be more convenient and may save time and money.
Due to advances in mechanical and chemical engineering, it is now possible to recycle paved surfaces in situ. Methods for in situ recycling are known as hot in-place recycling and cold in-place recycling and share the steps of degrading, rejuvenating, and compacting the old roads. One problem that may occur during recycling is that the aggregate in the old road is broken. Aggregate size is an important characteristic of a road because it influences stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance, and resistance to moisture damage. If the aggregate is broken while recycling, new aggregate may be required to ensure the asphalt's characteristics remain substantially unchanged. This may require the purchase of new aggregate and special or modified machinery to load and distribute new aggregate which may add to the time and money spent on recycling and reconstructing a road.